Hot gas generators have long been utilized for producing hot gas under pressure to operate engines of various types as well as for other purposes. In such hot gas generators, a carbonacious fuel is combusted with an oxidant to produce hot gases of combustion, and additional fuel may typically be introduced into the hot gases of combustion to be vaporized, or partly decomposed, or both. By so doing, the volume of hot gas can be increased while bringing the temperature of the combustion gas down to a temperature incapable of causing damage to the system in which the generator is used.
One difficulty in the operation and use of such hot gas generators is carbon buildup which results when the fuel is not completely oxidized and elemental carbon is formed within the combustion chamber of the generator. It is important to keep the internal walls of the combustion chamber hot so that the diffusion of carbon to the walls and adherence of carbon on the walls is minimized. Also, carbon buildup can be avoided by providing an excess of oxidant within the combustion chamber but this necessarily results in excessive consumption of oxidant during operation of the hot gas generator.
As a result, there is ordinarily a plentiful supply of liquid fuel in most cases. It is thus conventional practice to run a hot gas generator on the rich side so that all available oxidant is consumed during combustion to thereby minimize oxidant consumption. However, by so doing, the potential for carbon buildup is increased.
As pointed out in Parrin U.S. Pat. No. 1,828,784, issued Oct. 27, 1931, it is also desirable to cool the combustion chamber to prevent damage thereto by excessive heat from combustion occurring therein. Advantageously, this is accomplished by cooling the combustion chamber with fuel, but the fuel may get overly hot causing gumming up leading to rapid failure and, furthermore, the fuel starts to boil which makes fuel injector design difficult and causes serious control system instabilities. At lower power settings, this fuel overheating is particularly troublesome because the low pressure in the combustion chamber results in fuel boiling at lower temperatures.
From the foregoing, it should be clear that there are two fundamental considerations. First, the internal walls of the combustion chamber must be at a maximum temperature. Second, the heat flux through the internal walls must be minimal. In this manner, carbon buildup can be avoided while providing the necessary cooling.
As will be appreciated, carbon buildup is undesirable because it may interfere with heat transfer, but another problem resulting from carbon buildup is much more serious. Specifically, hot gas generators are frequently used to produce hot gases for driving turbine wheels. As carbon builds up, particles thereof typically break free and then flow with the hot gas through the turbine wheel. Unfortunately, particulate carbon erodes the turbine nozzles and the turbine wheels. Furthermore, carbon deposits can build up on the surfaces of the turbine nozzles and restrict the flow to cause performance losses.
The hot gas generators disclosed in commonly owned and co-pending applications Ser. No. 123,303, filed Nov. 20, 1987; Ser. No. 272,409, filed Nov. 17, 1988; and Ser. No. 324,806, filed Mar. 17, 1989 avoid many of these difficulties. Thus, they are recognized as highly advantageous. Nonetheless, improvements in terms of precisely controlling heat transfer from the combustion chamber through a liner and a wall while providing a simplified construction in a hot gas generator is also highly desirable.
As previously mentioned, it is a principal requirement to keep the combustor walls adjacent the flame hot so as to minimize carbon deposition. At the same time, heat loss from the flame must be kept to a minimum to avoid chilling of the combustor reaction and consequent performance loss. Furthermore, cooling air must pick up sufficient heat from the flame so that fuel flowing through the fuel injector does not freeze At the same time, it would be advantageous to preheat the cooling air prior to combustion to assure fast evaporation and early ignition of the fuel/air mixture at super cold temperature.
The present invention is directed to overcoming one or more of the foregoing problems and providing one or more of the suqgested improvements.